There is a long-standing need to monitor or inspect the condition of various hydrocarbon-based fluids such as aviation kerosene, lubricants and inks that are stored or flowing. Devices and techniques for the electrical measurement of parameters such as the conductivity, dielectric constant, and viscosity of fluids are known. Heretofore, most devices and techniques have used a sensor immersed in the fluid with electronic circuits to operate the sensor and produce the desired measurement. The physical properties of a sensor and the associated requirements of the electronic circuitry have typically restricted the direct measurement to that of only one parameter. In recent years, some techniques have been described that attempt to measure multiple parameters with one apparatus. For example, Visyx Technologies, Inc. has described a tuning fork resonator system driven at a spectrum of frequencies to generate data from which multiple fluid parameters are inferred by calculation. There is a need for a direct measurement of multiple fluid parameters that is not inferred by calculation. There is also a need for such direct measurements in real time, e.g. in the monitoring of fuel being fed to an engine as it is running.
There is also a long standing need to provide a device and system that measures both conductivity and dielectric constant, while simultaneously meeting other commercially important criteria such as: physical compactness, compatibility with low-cost, commercially-available electronics, cost-effective manufacturability, explosion resistance that meets recognized certification requirements, and commercially desirable levels of accuracy, resolution, stability and reliability in the measurements of the parameters of interest.
The invention should have the resolution to measure, for example, the level of water, absorbed or condensed as droplets, in a high impedance fluid such as lubricating oil, or, the ability to detect small quantities of drill mud contamination in hydraulic fluid.
D-2, Inc., the assignee of the present application, has previously developed methodologies for the measurement of electrical conductivity of aviation kerosene in a coaxial, two electrode sensor of the general type shown in FIG. 2, using circuitry of the general type shown in FIG. 3. D-2 has also adopted technologies using standard commercially available integrated circuits (IC's) to measure dielectric constant on a second pair of coaxial electrodes. These developments were effective for the measurement of aviation turbine fuel, but the complexity of a multiple electrode system does not lend itself to a low cost measurement of plural parameters of lubrication oil and like hydrocarbon-based fluids.
Low cost IC's that can measure conductivity or capacitance are known, e.g. for use with touch panels, but their operating characteristics such as their input capacitance range and capacitance off-setting capabilities are not suitable for use with this known hydrocarbon fuel sensor.